Field of the Invention--This invention relates to helicopter controls, and more particularly to offsetting certain negative speed stability effect, such as those that a large tail stabilizing surface has on the control response and stability characteristics of helicopters at cruise speeds.
Description of the Prior Art--As is known, the stability characteristics of helicopters are very complex, and vary widely from one helicopter design to the next. Just about every individual characteristic of a helicopter affects the stability one way or another.
Of course, certain of the design characteristics play more predominant roles in the stability than do others. The response of the helicopter, both to pilot controls and to inner-loop stability augmentation controls, is of course highly dependent upon the overall stability characteristics of the helicopter. In fact, both the response and the pilot feel of pilot-inputted control demands will vary not only with the design of the helicopter, but in any given helicopter, can be highly dependent upon the instantaneous operating conditions of the helicopter, such as airspeed, attitude, and loading.
There are certain well-known attributes of helicopter response which are desirable for flight stability per se, and are further desirable from the point of view of consistent response to pilot input, and consistent pilot reaction to operating conditions, responses, and external inputs to the aircraft flight conditions (such as wind gusts which affect airspeed or attitude of the helicopter). Examples include the desirability of positive angle of attack stability and positive speed stability, which combine to provide a desired positive relationship between longitudinal cyclic pitch stick position and airspeed (with other controls fixed), which is referred to herein as a positive static pitch trim gradient. To illustrate this feature, consider a helicopter operating at a rather steady cruise speed; a wind gust may impact the helicopter in a manner which alters its pitch attitude, thereby inducing a change in airspeed, or in a manner which may simply impact the aircraft with a sufficient component in the flight vector of the aircraft so as to alter its airspeed directly. Similarly, abrupt changes in air density acting upon the aerodynamic lift, either provided by the main rotor or by tail stabilizing surfaces, may alter the pitch attitude, and thereby provide an undesired input to airspeed. The pilot's natural reaction to a decreased airspeed or a decrease in pitch angle is forward motion of the longitudinal cyclic pitch stick from an initial trim position to cause the helicopter to rotate its nose down, followed by aft motion of the stick to arrest the nose-down rotation at the desired pitch angle for the required airspeed. Ideally, the stick should return to the same trim position in the case where the pilot is restoring a desired speed; and, ideally, the stick should be trimmed forward of the original trim position in the case where the pilot is purposefully increasing airspeed. This is referred to herein as a positive static trim gradient. A corollary to the stability achieved by a positive static trim gradient is the fact that the pilot is therefore provided with a correct relative feel in the cyclic pitch stick: that is, the increased force, which the pilot must provide to the stick to achieve trim at increasingly forward positions, provides a relative indication of speed and/or pitch axis inclination, on a continuous basis for any stick position, regardless of undesirable external inputs to the control system by the environment, or inadvertent pilot inputs.
Another known desirable helicopter flight control characteristic is the decoupling of collective pitch from the pitch axis of the helicopter: stated alternatively, it is desirable that increases or decreases in collective pitch will not cause nose up or nose down angular rotations of the helicopter in its pitch axis which would, in turn, upset the pitch trim.
As is known, a properly designed helicopter may be controlled in stable, maneuverable, descending flight after the loss of motive power to the rotor, in a mode called "autorotation". As stated very simply, the gravitational force allows the rotor to continue to rotate to provide aerodynamic lift, although descent ensues, speed stability is a function of body attitude which is, in turn, dependent on the size and incidence (or attack) angle of tail stabilizing surfaces. But, factors such as performance, center of gravity location and vibration may preemptively dictate size and incidence angle which result in negative speed stability. As long as the rotor is rotating, the cyclic pitch channels will function to permit controlling the attitude of the helicopter. In the conventional, older helicopters which did not have large horizontal tail stabilizing surfaces, loss of rotative power caused the helicopter to drop in essentially a level attitude, the pilot providing a small amount of aft cyclic stick to slow the rate of descent during autorotation, so as to permit a safe, flared landing in the safest available spot.
The design speed (cruise and maximum) of modern helicopters is ever increasing. At higher speeds, the achievement of flight stability is more difficult. When speeds are on the order of 100 knots or greater, stability may be improved with horizontal tail stabilizers which are large in contrast with older helicopters. As can be expected, however, this in turn alters other flight stability characteristics of the helicopter. For instance, a large tail surface can provide aerodynamic vertical lift to the tail which alters the dynamic center of the helicopter as a function of airspeed. Further, the angle of attack of the helicopter in contrast with the velocity vector direction of the helicopter can cause "weathervaning", which is a tendency for the tail surface to lift when it is not oriented along the velocity vector of the aircraft. Therefore, the response of the helicopter to pilot commands in the pitch axis may be influenced (or biased) at cruise airspeeds (eg, above forty or fifty knots) where these aerodynamic effects become significant. Furthermore, one consequence of larger tail surfaces and/or greater tail incidence angle is that changes in collective pitch tend to rotate the helicopter in its pitch axis, due to the aerodynamic lift of the tail (which is considerable at high speeds) remaining fixed, as the lift of the rotor is altered. For instance, in attempting to increase speed or to restore speed (in the examples hereinbefore), the "weathervaning" of a tail surface at high speed must be overcome by longitudinal cyclic stick positioning, such that a reverse static trim gradient exists. In autorotation, the aerodynamic lift to the tail surface will instantaneously cause the forward portion of the helicopter to drop more rapidly than the tail portion, whereas in the past helicopters without tail surfaces would tend to drop in a substantially level fashion. This is further compounded by the heavy loading of modern helicopter main rotors: that is, when rotative power is lost, the helicopter tends to descend at a greater rate than in the case of helicopters with lighter rotor loading. Once the helicopter starts to descend along a nose down glide path in autorotation, the "weathervaning" of the tail results in a greater nose-down pitch angle, accompanied by an increase in its descent speed.
It has been known in the art to provide pitch bias as a function of airspeed alone; however, this has resulted in loss of control margin and increase in sensitivity.